The use of highly reactive metals, such as magnesium in the treatment of molten metal (ductile iron, white iron, steel etc.) has been limited because of the strong pyrotechnical reaction which accompanies the introduction of such reactive materials into the molten metal. Magnesium and/or boron are therefor difficult to introduce into molten metal.
At 1,600.degree. C. the volume of 1 g of magnesium occupies 6.25 liters. At steel making temperatures the specific volume of magnesium vapor is 44,000 times the specific volume of molten steel.
This unfavorable specific volume factor for magnesium vapor to steel explains the extremely low solubility of magnesium in molten steel and generally in molten metal.
Also, at atmosphere pressure and 623.degree. C., magnesium vapors ignite violently in the present of oxygen to form cubic crystals of MgO, seen as a white smoke.
The strong affinity of magnesium for oxygen and other elemental species, such as sulfur, which results from its thermodynamic characteristics, makes it an ideal treatment agent in molten metal for the removal of inclusions which, when present, are detrimental to the tensile properties of the solidified metal. Magnesium is also useful in reacting with silica (SiO.sub.2) and maganese sulphate to remove these impurities by converting them to slag.
The intensity of the deoxidation reaction with magnesium in the molten metal is determined by the solubility of oxygen in the liquid. The solubility of oxygen in molten metal is a function of temperature according to the following equation: EQU log [%0]=-(6320/T)+2.734
Where the temperature T is expressed in degree Kelvin.
Therefore at 1,400.degree. C. the solubility of oxygen in molten metal is 0.019% and at 1,650.degree. C. it is 0.276%.
Magnesium's capacity for oxygen removal is not limited to the elimination of oxygen dissolved in the metal but also acts to reduce metal oxides (FeO, MnO, etc.) thereby lowering the total oxygen content of the molten metal.
The solubility of magnesium is a function of temperature according to the equation. ##EQU1##
Where P is the partial pressure of magnesium in kg per square centimeter and T the temperature expressed is in degrees Kelvin.
Due to the above considerations, and primarily because of its low solubility, it is very difficult to introduce enough magnesium, into molten ferrous alloys at the temperature of interest, for effective deoxidation, desulfurization and inclusion control.
The kinetics of the general deoxidation/desulfurization reaction in ferrous alloys using the injection procedure are determined by the following factors:
1. The specific rate based on the mechanism of reaction. PA0 2. Contact time between injected species and the reacting species, solid or vapor, in the liquid alloy. PA0 3. The degree of dispersion of the injected species in the molten alloy. PA0 4. The efficiency and duration of the reactions as determined by the influence of slag, refractories and the atmosphere. PA0 5. A very low chemical concentration gradient of dissolved sulfur and oxygen in the molten metal. PA0 1. Maximum surface contact for reaction between magnesium and metal by formation of extremely small magnesium vapor bubbles. PA0 2. Disrupted flow of magnesium vapor to increase both surface area (avoidance of piping effect) and exposure time. PA0 3. Increase in total contact time between magnesium vapor and the molten metal achieved by increasing the penetration depth of the injected magnesium in the molten metal. PA0 4. Increase in ferrostatic pressure by increasing the ladle height to diameter ration (also the increase in H/D causes more molten metal to be exposed a longer period of time to the magnesium vapors).
When magnesium is injected according to the present invention into molten ferrous metal, instantaneous vaporization occurs and a high volume of magnesium vapor is formed, which creates a turbulent stirring of the liquid and thereby creating convection inducted microdiffusion. Resulting from the magnesium injection treatment, a rapid and homogenization of the components in the molten metal will occur, favoring the kinetics of the deoxidation and desulfurization reaction.
The desulfurization process occurs at the interface between the magnesium vapor and the dissolved sulfur, and is enhanced by the higher surface activity of the sulfur in the vicinity of the vapor. At the interface between the vapor and the molten metal the ratio a.sub.S /a.sub.Fe is several times higher than a.sub.S /a.sub.Fe at locations remotes from the interface, where a.sub.S is the activity of the sulfur in the molten metal and a.sub.Fe is the activity of the molten iron.
The rate of removal of the solid reaction products (MgO, MgS) is determined by the difference in densities of the reaction products (2.8 gm/cm.sup.3 for MgO) and the molten metal (7.3 gm/cm.sup.3) and the viscosity of the molten metal.
The transport mechanism in turbulent flow (produced by magnesium injection according to the present invention), is more rapid than in the normal diffusion (non turbulent) process. The mass transport of the reacting increments to the effective interface is a component part of whole reaction process and may be the rate determining step.
To obtain the maximum deoxidation and desulfurization effect it is necessary to react the total amount of magnesium with the molten metal. Factors which enhance the total reaction are:
The present invention provides a unique combination of procedure and equipment to introduce into molten metal, preferrably nearly pure (50-100%) magnesium projectile (bullet) for the purpose of cleaning and refining the metal.
The amount of magnesium introduced into the molten metal is dependent on the specific alloy chemistry, the level of cleanliness required to attain superior mechanical properties and further the anticipated magnesium recovery.
A series of emperical tests have been performed on various grades of steel, ductile iron and white irons, to demonstrate the efficiency of the invention, favorable economic and the superior properties attained.
In carrying out the introduction of magnesium into the molten iron alloy materials, I employ a pneumatic gun which injects one or more bullets into the molten metal. Each bullet has a magnesium or ferro-magnesium body and pointed steel tip or nose with or without slots. The pneumatic gun is disposed over the molten metal in a container (ladle). The gun has a barrel, disposed along a vertical axis or at a prescribed angle to the surface of the molten metal so as to direct the bullet, containing additive metal, magnesium or boron for example, into the molten metal, at a velocity sufficient that the additive metal is submerged and progressively disintegrates and dissolved as it passes through the molten metal and the steel nose is thereafter melted and becomes a part of the molten alloy. The gun has a straight, upright, tubular magazine in which the bullets are stacked, end-to-end with the lowermost bullet being supported solely by a solenoid actuated gate, within the chamber of the gun. A second solenoid actuated gate in the breach of the barrel, received and supports a bullet in the chamber for discharge down the barrel.
Compressed air, fed through a pressure regulator and a solenoid valve, is introduced into the chamber as the barrel gate releases the bullet, so that the compressed air propels the bullet down the barrel, through the splash guard, and into the molten metal.
There are several embodiments of bullets which I have found suitable for use in the gun, one, a magnesium bullet is shown, having a cylindical body with a semi-spherical steel tip or nose. Another has a tapered steel tip or nose and still another a conical steel tip or nose. The sides of the nose and/or the body can have axially extending slots or grooves for guiding the bullet in its path through the molten metal and for providing increased reaction area.
Hollow, ingot carrying, bullets are also shown. These bullets have hollow cylindrical bodies with steel tips at both ends, the additives being confined within the body. Such bullets are used primiarily for introducing boron into molten metal. The body can be steel tubing, aluminum tubing copper tubing or the like.
The process of the invention includes producing bullets with pre-selected additive metal contained in each bullet and shooting one or more of these bullets preferably vertically into the molten metal in the ladle. The magnesium or boron which is introduced by such a procedure is thus progressively dissolved in the molten metal.
The boron addition, when shot beneath the surface of molten metal according to the above outlined procedure produces under cooling in the alloy which promotes smaller intricate crystals or carbides which produce superior tensile strength in white irons and steel.